The invention concerns the electrical discharge machining of the surface of a recess in a workpiece electrode by means of a tool-forming electrode. For the sake of simplification, these electrodes will hereinafter be referred to separately as "workpiece" and "tool" and jointly as "electrodes". The invention specifically concerns machining in which the electrodes are moved relative to one another both in the direction of an axis of penetration of the tool in the workpiece and in a plane perpendicular to this axis.
It is known to machine the lateral faces of a workpiece by moving the electrodes relative to one another with a translational movement, or orbital motion, in a plane perpendicular to the axis of penetration as disclosed in U.S. Pat. No. 2,773,968, and vary the amplitude of this movement as a function of the relative displacements of the electrodes along said axis as disclosed in U.S. Pat. Nos. 3,135,852, 3,539,754 and 3,809,852, which displacements are controlled to maintain given sparking conditions in the machining zone comprised between the electrodes.
All those methods of machining result in machining of the lateral surfaces of the workpiece to a shape different from the shape of the tool. Another result is that the active lateral surface of the tool is displaced substantially parallel to the machined surface on the workpiece, such that there is no increase of the gap when the tool electrode is retracted. The machined shape, which is for example determined by the factor of proportionality between the amplitude of the translational movement and the axial displacement of the tool, corresponds to the shape of an envelope of the trajectories during the cyclic translational movements. A similar method, in which an orifice of increasing conicity is machined with a tool in the form of a flat disc, is described in W. German Published Patent Application (DOS) No. 2,238,698.
The invention concerns another method of machining which enables simultaneous machining of the front and lateral surfaces of a recess to the same shape as the tool. Up to the present, the finishing machining of a recess by successive passes with a single tool could be carried out using the method described in French Pat. No. 1,274,953. This method consists in making the tool penetrate in the workpiece during a machining pass by a relative transverse translational displacement of constant amplitude, and then repeating this operation, in a following pass, after having increased the amplitude of this movement, thus usually causing at least a partial withdrawal of the tool from the workpiece. This method of machining produces a great local wear of the tool and necessitates a control of the frontal penetration of the tool in the workpiece in addition to control of the amplitude of the translational movement.
The invention provides a new method which enables: (a) avoidance of successive withdrawals of the tool from the workpiece, (b) elimination of local wear of the tool, and (c) control of the three-dimensional progression of machining, using a single device for measuring the amplitude of the translational movement.
This new method of machining is characterized in that one of the electrodes is made to penetrate into the other to a depth such that sparking is produced on the frontal and/or lateral parts of said surface, then the electrodes are moved relative to one another with oblique translational movements along at least one generatrix of a surface of revolution of increasing section in said direction of penetration, these oblique translational movements being obtained in a manner known per se by a rigid connection between rectilinear translational movements along said axis and translational movement in said plane and controlled, in a manner known per se, so as to maintain given sparking conditions in a machining zone comprised between the electrodes, this zone simultaneously extending on said frontal and lateral parts of said surface during at least a part of the finishing machining.
When, for example, said surface of revolution is in the form of a cone whose apex forms a right angle, control of the axial and radial advance of machining is obtained simply by a limitation of the amplitude of the translational movement to a predetermined value. This limiting value corresponds to a given point of one of the generatrices of this cone in the case of linear translation, or to a circle inscribed on the surface of this cone when there is a cyclic translation. In both cases, the rapidity and precision of machining are greater than those that would be obtained using the known machining methods.